The enzyme monoamine oxidase (MAO) is central to the metabolism of the putative amine transmitters, serotonin, epinephrine, norepinephrine, dopamine, octopamine, as well as other important endogenous amines such as tyramine, tryptamine, phenylethylamine, etc. Even though MAO is obviously and intimately involved in the turnover and regulation of these important amines, controversy still quietly rages concerning whether it is the same enzyme in different tissues, whether it exists in multiple forms, whether it is a single enzyme with independent active sites having certain substrate specificities, or different subunits with single sites. Additionally, it has been suggested, and equally disputed, that this enzyme may be a genetic marker for vulnerability to schizophrenia, as well as play an important role in the affective disorders, particularly, and in mental disease generally. The research we have accomplished and plan to do using electron spin resonance (ESR) probes of the active site of the enzyme as well as determining the catalytic activity of the site, addresses itself directly to these questions and problems. Not only are we obtaining, and expect to continue to obtain, further clarifying basic information concerning the characterization of this enzyme as well as its mechanism of action with respect to substrates and inhibitors, but we are also applying this basic knowledge to the human platelet enzyme and to states of mental illness like schizophrenia and the affective disorders. It is hoped that this work will therefore add to our knowledge and understanding of the etiology of these disorders and to better methods for their treatment.